Energy harvesting devices, such as devices or systems that produce electrical current by harvesting energy from renewable resources, are increasingly used to provide power pack charging energy to rechargeable devices. For example, solar cells harvest energy from sunlight or other light sources and produce electrical current. Another example is wind power generators that harvest energy from the wind to turn mechanical electrical generators that provide electrical current.
Although the advantages of using renewable energy are well known, energy harvesting devices that harvest energy from many types of renewable energy sources are subject to the energy output fluctuations and intermittent availability of their originating energy sources. For example, sunlight available to a solar cell varies during the day due to cloud cover and other factors, and is completely unavailable at night. Wind powered generators are also subject to fluctuating output based on varying winds, and may have long periods of no output due to long periods of calm winds.
Some energy harvesting systems accommodate periods of no output due to the unavailability of the energy source being harvested by using rechargeable batteries to store harvested energy. The energy stored in these rechargeable batteries is used when the energy harvesting device is not harvesting energy. Such devices, particularly less expensive device charging stations, switch between drawing energy from either the energy harvesting device or the rechargeable battery. In an application of a rechargeable device charging station, the energy harvesting device generally has an electrical current output that is below the power that can be used to charge the batteries or other power packs within the rechargeable device. Using only the electrical current available from the energy harvesting device to recharge a battery of a rechargeable device often results in long charging times.
The usability of charging stations that draw energy from intermittent and fluctuating energy harvesting devices is limited by the low output electrical current of the energy harvesting devices relative to the maximum charging current that can be accepted by a device being recharged.